The present disclosure relates to digital photocopying and printing on print media sheets and particularly such processes in which the media sheets are fed serially from at least one tray or feeder and may traverse any of several chosen paths through one or a multiplicity of marking engines. In such photocopying and printing, the media sheets typically pass through a myriad of nip rollers and gates where the transport speed may be varied and the sheets are directed around numerous bends and the sheets may also be inverted for duplex printing or printing on both sides of the media sheet.
Heretofore, in digital photocopying/printing and particularly with electrostatic photocopiers, the media sheet path is chosen by the electronic programmer once the user has inputted the print job requirements. The sheets are fed and transported through the marking engine(s) with occasional or very limited sheet position readings by sensors located along the sheet path for providing a basis for correcting the timing of the media sheet feed into the marking engine(s) and the progress of the media sheets through the marking engine(s). The progression of media sheets through the marking engine(s) has thus essentially been accomplished by open loop control.
Where media sheets progress through a complicated transport path of multiple nip rollers, bends, and gates, variations in the path length due to varying properties of the print sheet media such as varying length, variations in the velocity on the surface of the nip rollers, variations in the bends through which the sheet traverses have allowed sheet positioning errors to compound thereby resulting in collisions, mis-registrations and jamming. Problems of this sort have been particularly acute in arrangements where large documents are to be printed at high speed in parallel paths through multiple marking engines. The combination of high sheet velocity and extended complex sheet paths are intolerant of substantial variations in the timing of the sheet position along the path in order to prevent collisions, mis-registration and jamming.
Thus, it has been desired to provide a way or means of improving the media sheet control and transport through marking engines in digital printing in a manner which eliminates or minimizes mis-registration and jamming.